U.S. Pat. No. 4,831,484, by co-inventor Bruch hereof, the disclosure of which is hereby incorporated by reference, shows a safety barrier circuit which has a voltage limiting circuit using voltage limiting Zener diodes, combined with a fuse which disconnects the output circuit if the voltage at the input to the safety barrier, which would be reflected at the output terminals of the safety barrier, exceeds a threshold limit determined by the Zener diode or Zener diodes.
It has been found that, if the input voltage exceeds the Zener threshold voltage only slightly, a substantial current rise may occur. The increase in current level may be substantially above the current which can be applied at the output terminals with maximum permissible input voltage. The fuse then burns through, disconnecting the output voltage. The maximum permissible output current, even under short circuit conditions, considering the customary dimensioning of the safety barrier, is below the burn-through or, rather, the melt-through current of the fuse. Normally, therefore, the fuse will burn or, rather, melt through only in case of excessive voltage at the input to the safety barrier.
Safety barriers are so constructed that the fuse, together with the remaining structural elements, is encapsulated in a housing, and inhibiting access to the components thereof. This is required for safety reasons, and to prevent tampering with the safety barrier. If the fuse burns through or melts through, the entire safety barrier becomes unusable. To protect the rather expensive safety barrier element, it has been proposed to place an additional fuse in series with the input of the safety barrier. That additional fuse was dimensioned to have a fusing current rating smaller than the threshold current resulting in melt-through of the fuse which is encapsulated in the safety barrier. This additional fuse was intended to protect the encapsulated fuse in case of over-voltages, so that one could merely exchange the external fuse and then place the barrier again in operation, without interfering with protective regulations which determine the structure and installation of safety barriers.
It has been found that an external protective circuit including a fuse, connected to the safety barrier, did not provide the desired protection for the fuse encapsulated within the safety barrier. Over-voltages or surges may occur due to inductances in the circuit of which the safety barrier is a part and, for example, in the connecting lines to the safety barrier; then both fuses will burn through although the external fuse and the internal encapsulated fuse may have substantially different current ratings. When the internal, encapsulated fuse burns out, the entire barrier becomes useless and must be exchanged. Since barriers are expensive elements, and fuses are cheap, exchange of the entire barrier merely because the encapsulated fuse burns out is economically undesirable.
The Invention. It is an object to provide a protective circuit for a safety barrier so that, under external over-voltage or surge on pulse voltage conditions, the internal fuse forming the final protection for a load in a potentially hazardous location is protected, thereby avoiding replacement of an entire expensive circuit structure when only an inexpensive, normally replaceable element thereof burns through.
Briefly, an externally accessible current interrupting element, such as a circuit breaker, fuse or the like, is connected in series with the safety barrier, and, additionally a second voltage limiting circuit is connected between the input of the voltage limiting circuit inherent in the safety barrier and the internal fuse of the safety barrier; the external current interrupting element, such as a fuse or circuit breaker, is connected in advance of the second voltage limiting circuit and the current interrupting element is set to have a current interruption threshold which is at least as high, and may be higher than the current interruption threshold of the fuse encapsulated in the safety barrier.
The external current interrupter in combination with a second voltage limiting circuit connected electrically in advance of the internal encapsulated fuse of the safety barrier permits a construction in which the external current interrupter such as a fuse, is accessible, and hence replaceable. The encapsulated fuse within the safety barrier protects the output terminal and, to comply with safety requirements, is arranged so that it is inaccessible. Thus, it is possible to reset the circuit through the safety at low cost barrier without interfering with safety regulations since the second, externally accessible interrupting element will respond to disconnect excess voltages from the output terminal. It is not specifically necessary, even, to prevent insertion of an externally accessible fuse with a current level which is higher than the recommended fusing level; the worst that could happen is that the internal, encapsulated fuse will burn through. Even if the fuse or interrupter terminals which are externally accessible are short-circuited, the encapsulated internal fuse will respond since it of course finally sets the limiting value of the safety barrier.
In accordance with a preferred feature of the invention, the externally accessible fuse has the same or only slightly higher current response level as the internal, encapsulated fuse. Even if the second, externally accessible fuse has the same current response level as the encapsulated fuse, a larger current will flow through the externally accessible fuse than through the encapsulated fuse, if the voltage limiting circuit associated with the externally accessible fuse as well as the internal voltage limiting circuit of the safety barrier have the same voltage threshold level. The second, externally accessible fuse will thus reliably respond before the internal encapsulated fuse, thus protecting the safety barrier without, however, interfering with its safety functions.
The series circuit which includes the second, accessible fuse and the voltage limiting circuit associated therewith operates as a clamping circuit for the subsequent voltage limiting circuit of the barrier unit. Thus, the response characteristic of the accessible fuse is not critical and can be selected within wide limits; for example, the second accessible current interrupting element can be a fuse, a circuit breaker, or a positive temperature coefficient resistor of higher current rating or slower response time than the encapsulated interior fuse of the safety barrier.
The safety barrier can be designed for d-c operation of predetermined polarity, or for connection to a network of varying polarities, for example an a-c network. In accordance with a preferred feature of the invention, the voltage threshold values of the internal voltage limiting circuit of the barrier as well as of the voltage limiting circuit associated with the accessible current interrupting device are of equal nominal dimension; different dimensioning, however, may also be possible, for example the threshold level of the voltage limiting circuit associated with the accessible current interrupting device can be designed for a lower voltage level than the voltage limiting circuit of the barrier itself.